This invention relates to a fibre optic sensor and in particular to a cyclic or Sagnac type ring interferometer for sensing mechanical or thermal disturbances, for example.
The Sagnac or cyclic interferometer is one of several types of well known interferometer. Initially, such interferometers used a plurality of mirrors arranged so as to reflect light beams around a loop. Subsequently, the advent of laser diodes and fibre optics has allowed the development of Sagnac interferometers having a much longer path length. A light source can be injected into a wave guide which is formed by a fibre optic in the shape of a loop.
In recent years, a number of proposals have been put forward to locate mechanical or thermal disturbances occurring at any non-central location in the loop. One such arrangement is shown in xe2x80x9cA novel distributed optical fibre sensing system enabling location of disturbances in a Sagnac loop interferometerxe2x80x9d, by J. P. Dakin, D. A. Pearce, A. P. Strong, and C. A. Wade in Proceedings SPIE, vol 838 (1987). The technique is based upon the counter-propagating nature of light in the Sagnac interferometer. When a phase perturbation, xcfx86, occurs at a distance z from the sensor loop centre, it phase-modulates the light travelling in one direction before light travelling in the other. This results in a net phase modulation, xcex94xcfx86, between the two returning counter-propagating wavetrains which interfere, when combined at the output of the loop. As set out in the above referenced paper by Dakin et al, xcex94xcfx86 is given by       Δ    ⁢          xe2x80x83        ⁢    φ    ⁢          xe2x80x83        ⁢          (      t      )        ≈                    2        ⁢                  xe2x80x83                ⁢        z                    V        g              ⁢          xe2x80x83        ⁢                            ⅆ          φ                ⁢                  xe2x80x83                ⁢                  (          t          )                            ⅆ        t            
where Vg is the group velocity of the guided light.
In initial arrangements, the value of dxcfx86/dt was found by interfering a fraction of the light that had travelled in one direction in the Sagnac loop, with light directly from the source. In order to reduce phase noise from frequency fluctuations of the source, the latter was suitably delayed via a fibre loop to form a balanced path fibre Mach-Zehnder arrangement.
More recently, various architectures using twin-Sagnac configurations have been suggested, to avoid the need for accurately balanced paths. Some of these architectures permit the location of disturbances over sensor loop lengths of up to 800 m, although only under laboratory conditions. For example, twin-source (wavelength multiplexed) devices are shown in the papers xe2x80x9cDual wavelength Sagnac-Michelson distributed optical fibre sensorxe2x80x9d, Proceedings SPIE pp2838-2834 (1996) and xe2x80x9cA distributed dual wavelength Sagnac sensor impact sensorxe2x80x9d, Microwave and optical technology letters, vol 17, No 3, pp 170-173, (1998), by S. J. Spammer, A. A. Chtcherbakov, and P. L. Swart. Other intrinsically lossy arrangements with directional 3 dB couplers and twin detectors are shown in, for example, S. J. Spammer, P. L. Swart, A. Boosen, xe2x80x9cInterferometric distributed fibre optical sensorxe2x80x9d, Applied Optics, Vol 35, No 22, pp 4522-4523, (1996), E. Ronnekleiv, K. Blotekjaer, K. Krankes, Distributed fibre sensor for location of disturbances, Proceedings 9th OFS, PD7, (1993) X. Fang, xe2x80x9cA variable loop Sagnac interferometer for distributed impact sensingxe2x80x9d Optics letters, vol 21, No 6, (1996), and in U.S. Pat. No. 5,046,848.
The minimum theoretical loss of a dual Sagnac system with 3 dB couplers is 24 dB in each Sagnac.
According to the present invention there is provided a Sagnac interferometer for sensing a disturbance, the interferometer comprising: a light source arranged to generate a light signal over a range of wavelengths xcexs; an optical receiver arranged to receive light signals generated by the light source; a fibre optic loop sensor in optical communication with both the light source and the optical receiver; first optical splitter means, in optical communication with the said source, for spectrally slicing the light signal received from the source into first and second split signal channels, the first split signal channel having a range of wavelengths centred at xcex1, and the second split signal channel having a range of wavelengths centred at xcex2, wherein both xcex1 and xcex2 are subsets of xcexs and wherein the electromagnetic energy contained in the range of wavelengths centred at xcex1 substantially does not overlap with the electromagnetic energy contained in the range of wavelengths centred at xcex2; first optical combiner means, in optical communication with the optical detector; the first split signal channel being defined along a first optical path between the optical splitter means and the optical combiner means via the loop sensor, and the second split signal channel being defined along a second optical path, different from the first optical path, between the optical splitter means and the optical combiner means via the loop sensor; the first optical path including a first optical phase angle modulating means arranged to modulate the phase angle of light in the first split signal channel at a first frequency, the first optical path having a total optical path length whose centre lies at a first non-central location around the loop sensor; the second optical path including a second optical phase angle modulating means arranged to modulate the phase angle of light in the second split signal channel at a second frequency different from the said first frequency, the second optical path having a total optical path length whose centre lies at a second non-central location different from the said first location around the loop sensor; the first optical combiner means being arranged to combine phase modulated light in the first and second split signal channels into a composite signal; the optical receiver being arranged to receive the composite light signal and to extract therefrom signal variations arising from light traversing the said first and second optical paths, whereby the distance of the disturbance around the loop sensor may be determined on the basis of the said signal variations.
The present invention provides a Sagnac loop interferometer architecture having two separate optical paths but with a single light source and a single detector. The resultant architecture has significantly lower minimum theoretical losses and is also cheaper to produce than previous arrangements which use multiple light sources and/or detectors.
Preferably, the first split signal channel defines the first optical path between the optical splitter means and the optical combiner means by traversing, in order, the first optical phase angle modulating means, the sensor loop and a first length of fibre defining a first delay loop such that the centre of the first optical path is offset relative to the centre of the sensor loop.
In that case, the second split signal channel may define the second optical path between the optical splitter means and the optical combiner means by traversing, in order, a second length of fibre defining a second delay loop, the sensor loop and the second optical phase angle modulating means, such that the centre of the second optical path is offset relative to the centre of the sensor loop in the opposite direction to the direction of offset of the centre of the first optical path.
The optical splitter means may be, for example, a wavelength division multiplexer (WDM). The light source may be a single superluminescent fibre source pumped by a laser diode producing broadband, low coherence length light. The detector may be a single p-type intrinsic n-type (PIN) semiconductor photodiode, and the first and second phase modulators may for example be piezo electric (PZT) devices.
Preferably, the first frequency of the first phase modulator and the second frequency of the second phase modulator are each Eigenfrequencies of first and second interferometer loops defined between the light source and the detector via the first and second optical paths respectively.
The optical receiver may comprise, for example, a photodiode optical detector, a low noise preamplifier and suitable signal processing software or hardware. This allows those amplitude modulated components corresponding to the variation in the optical signal amplitude to be extracted from the composite detected signal. The variations in the optical signal amplitude in turn arise from the disturbance as well as the phase modulations imparted by the phase biasing means.
Preferably, the distance z on the loop sensor at which the disturbance has occurred is determined by calculating the ratio of the fundamental and second harmonic components arising from the light which was phase modulated at the first frequency to the fundamental and second harmonic components arising from the light which was phase modulated at the second frequency
In that case, the ratio may be given by:   ratio  =                    J        1            ⁢              xe2x80x83            ⁢              (                  x          ⁢                      xe2x80x83                    ⁢                      a            1                          )            ⁢              xe2x80x83            ⁢                        J          2                ⁡                  (                      x            ⁢                          xe2x80x83                        ⁢                          a              2                                )                    ⁢              xe2x80x83            ⁢              (                  L          +                      L            1                    -                      2            ⁢                          xe2x80x83                        ⁢            z                          )                            J        1            ⁢              xe2x80x83            ⁢              (                  x          ⁢                      xe2x80x83                    ⁢                      a            2                          )            ⁢              xe2x80x83            ⁢                        J          2                ⁡                  (                      x            ⁢                          xe2x80x83                        ⁢                          a              1                                )                    ⁢              xe2x80x83            ⁢              (                  L          -                      L            2                    -                      2            ⁢                          xe2x80x83                        ⁢            z                          )            
where x is the bias modulation depth, a1 and a2 are constants, L is the length of the sensor loop, L1 is the length of the first delay loop, L2 is the length of the second delay loop, and J1 and J2 are first and second order Bessel functions respectively.
In further aspects of the present invention, a new configuration for an optical fibre disturbance location system is provided, based on a Sagnac loop, where light from a single broadband source is split into two separate spectral components of different wavelength, and said components of the light are directed, using wavelength-dependent splitting elements (for example, optical fibre based wavelength division multiplexer components) such that the light travels through two separate Sagnac loops, each path determined according to the wavelength region of the said components of the light, and where the light in each wavelength region is phase-modulated at a different frequency by each of two phase-biassing element, each operating at a different modulation frequency and at point in the fibre path unique to one of each of the two selected wavelength-dependent paths, and where each wavelength component passes through different optical delay paths, either before or after it passes through a common sensing section of fibre or fibre cable, where detection of disturbance is desired and where eventually all the output light from the two dependent paths is then combined onto a single optical detector, and the separate outputs from the two loops are then decoded by detection of the modulation components at the phase-biassing frequencies of the phase modulators.
The invention also extends to a Sagnac loop, where two separate optical paths are arranged between a single light source and a single optical detector and where the signals travelling around these separate paths are separated, after the initial optical detection, by virtue of frequency-dependent electronic detection of the output of the optical detector, using separate electronic lock-in amplifiers each driven by different electronic reference signals, each of said reference signals corresponding to the different phase modulation components and the second harmonic of the phase modulation components applied to the optical signals travelling in each of the two different optical paths.